Sintering in Mesh Belt Furnace: Powder Metallurgy Process Control from Delube to Sinter
Powder metallurgy (PM) parts are made by pressing metal powder into a shape and then heating the pressed part in a controlled atmosphere to bond the powder particles together. The heating step is called sintering, and the mesh belt furnace is the workhorse of the PM industry for high-volume production of small to medium parts. Sintering is a precise process. Get the temperature, the time, or the atmosphere wrong, and the parts come out weak, brittle, or distorted.
Here is how the PM sintering process actually works in a mesh belt furnace.
Start with the parts and the alloy.
PM parts start as metal powder - typically iron-based, with copper, nickel, and graphite added for alloying. The powder is mixed with a lubricant (typically 0.5 to 1.5 percent of a wax or a fatty acid like zinc stearate), pressed into a die at 25 to 50 tons per square inch to form the green part, and then loaded onto the mesh belt for sintering.
Common PM alloys include:
- F-0000 (pure iron, no alloy additions)
- F-0005 (0.5 percent graphite for mild strengthening)
- F-0008 (0.8 percent graphite for higher strength)
- FC-0208 (2 percent copper, 0.8 percent graphite for higher density parts)
- FN-0205 (2 percent nickel, 0.5 percent graphite for toughness)
- FL-4605 (1.5 percent Mo, 0.5 percent graphite for high strength)
Each alloy has its own sintering temperature range, its own atmosphere requirement, and its own dimensional change characteristics. The furnace has to be set up for the alloy being run.
The sintering process has three stages: delubing, sintering, and cooling.
The delubing stage is first.
The green part contains 0.5 to 1.5 percent lubricant. The lubricant has to be removed before the part reaches the sintering temperature, otherwise it vaporizes inside the part, creates internal pressure, and cracks or blisters the part.
The delubing stage is typically the first 0.5 to 1.5 meters of belt travel in the furnace, at a temperature of 400 to 600 degrees C. The lubricant vaporizes and burns off, and the combustion products flow out of the furnace with the exhaust gas. A small amount of excess air is introduced in the delubing zone to support the combustion of the lubricant vapor.
The delube zone is the dirtiest part of the furnace. The lubricant vapor condenses on cold surfaces, particularly at the entry end of the furnace. The condensate is a waxy, sticky material that has to be cleaned out periodically. Some furnaces have a delubing hood with a dedicated exhaust that captures the vapor and routes it to an afterburner.
The sintering stage is the heart of the process.
After delubing, the part enters the sintering zone at 1100 to 1300 degrees C. The exact temperature depends on the alloy:
- Iron-based PM: 1120 to 1150 degrees C for standard parts, 1250 to 1300 degrees C for high-temperature sintering
- Stainless steel PM: 1200 to 1300 degrees C
- Bronze PM: 800 to 900 degrees C
- Aluminum PM: 590 to 620 degrees C (well below the iron-based range)
The sintering zone is typically 2 to 4 meters of belt travel, with a residence time of 15 to 45 minutes depending on the belt speed. During the sintering time, several things happen in the part:
- The iron particles bond to each other through solid-state diffusion, creating necks between particles that grow with time
- The copper in the alloy (if present) melts and wets the iron particles, creating liquid-phase sintering that accelerates densification
- The graphite dissolves into the iron, forming pearlite or other microstructures as the part cools
- The part shrinks (typically 0.1 to 0.5 percent linear dimensional change) as the porosity decreases
The atmosphere in the sintering zone is critical. The atmosphere is typically a reducing gas (hydrogen, dissociated ammonia, or endothermic gas) that prevents oxidation of the iron and the alloying elements. Hydrogen and dissociated ammonia (DA, which is 75 percent H2 and 25 percent N2 from cracking ammonia) are the most common atmospheres for PM sintering.
The dew point of the atmosphere is a key quality variable. A dew point above -40 degrees C causes surface oxidation on the iron, which prevents proper sintering at the surface and reduces the strength of the part. The dew point is typically held at -40 to -60 degrees C for high-quality PM parts.
The cooling stage is the final zone.
After the sintering zone, the part cools in a controlled atmosphere through a cool-down zone. The cool-down rate affects the final microstructure - faster cooling gives finer pearlite, higher strength, and lower ductility. Slower cooling gives coarser pearlite, lower strength, and higher ductility.
Most PM parts cool at 0.5 to 1.5 degrees C per second through the pearlite transformation range (700 to 600 degrees C). The cool zone is typically 2 to 3 meters of belt travel, with the atmosphere still flowing to prevent oxidation. The cool zone can be a water-jacketed section, a forced-air section, or a long insulated tunnel.
A common PM line has a slow cool zone for parts that need coarse pearlite and a fast cool zone for parts that need fine pearlite. The belt is routed through one or the other depending on the part spec.
The atmosphere flow through the sintering furnace is counter-current to the belt.
The atmosphere enters at the cool end (where the parts exit) and flows toward the hot end (where the parts enter). This counter-current flow protects the hot end from air ingress, recovers some heat in the cool zone, and carries the lubricant vapor from the delubing zone out the exhaust.
The atmosphere flow rate is set to maintain a slight positive pressure throughout the furnace (typically 0.5 to 5 mbar above atmospheric) and to flush out any air that leaks in. A flow of 5 to 20 cubic meters per hour per square meter of belt area is typical.
The mesh belt in a sintering furnace is a special design.
The belt has to survive the 1100 to 1300 degrees C sintering temperature. Standard 314 stainless runs out of life quickly at these temperatures. Inconel 600 or 601 is the standard belt material for high-temperature sintering. The belt is typically a balanced weave with a heavier wire than a heat treatment belt, to handle the higher temperature and the longer residence time.
Belt life in a sintering furnace is typically 3 to 12 months, depending on the temperature, the atmosphere, and the parts loading. The end-of-life mode is usually wire breakage from thermal fatigue and corrosion.
A common quality issue in PM sintering is dimensional variation.
The shrinkage during sintering depends on the powder characteristics, the pressing pressure, the sintering temperature, and the sintering time. Variations in any of these cause variations in the part dimensions. The typical dimensional tolerance for as-sintered PM parts is plus or minus 0.3 to 0.5 percent. Tighter tolerances require a sizing operation (re-pressing) after sintering.
A second common quality issue is poor surface finish.
The surface of a sintered PM part depends on the powder size, the pressing condition, the sintering atmosphere, and the cooling rate. A coarse powder gives a rough surface. A high dew point gives surface oxidation that discolors the part and reduces corrosion resistance. A high cooling rate gives a fine pearlite that is harder but more brittle.
Author: MONTE INTELLIGENCE PM sintering furnace team. For sintering process audits and atmosphere system design, contact helenxu@cnlymonte.com.
